In molding, particularly blow molding articles such as bottles, the plastic is first heated to soften it and then extruded from an annular orifice into a tubular ribbon, commonly called a parison. The orifice points vertically downward so that the parison flows from the orifice. Relatively short strips or sections of the parison are then severed, inserted into a blow mold and expanded to the desired shape by blowing air or a gas into the parison until it is in firm contact with the walls of the interior mold cavity. Thereafter, the mold is opened and the bottle is discharged therefrom mechanically or by gravity for transport to an inspection station for packing and shipping. In many applications the transport is accomplished by providing a horizontal band conveyor beneath the mold which receives the discharged bottles and transports them away.
The molding machine must occasionally be shut down for short periods of time, often for no more than for 15 to 30 minutes, to make minor adjustments or repairs. When the plastic material is polyvinylchloride (PVC) this presents a problem because PVC must be kept in continuous motion in the extruder to prevent it from decomposing within the extruder. Consequently, during such downtimes, the extruder continues to operate which results in an accumulation of an initially hot and softened and thereafter hardened plastic mass. This mass is difficult to handle and cannot be ground into pellets in conventional grinders for subsequent re-use. Instead, the mass must either be discarded or hand sawed into small pieces that can be handled by the grinders. Either alternative is relatively expensive because of the ever increasing cost of petroleum based materials such as PVC or the high cost of manual labor.
To eliminate such waste hot melt grinders have been devised which receive the heat softened PVC parison and immediately grind into small pellets. Hot melt grinders are large and bulky and relatively expensive. Due to the large expense a single hot melt grinder is usually provided for several molding machines. When the machine is shut down the hot melt grinder is moved to the affected machine so that it can receive and grind the hot parison. The bulkiness of the machines block the access to the extruder and the mold and renders machine adjustments or repairs more difficult or impossible to perform. Moreover, the grinder is usually at the wrong place and must be moved substantial distances to the affected machine. This is time-consuming and permits the accumulation of parison which can no longer be introduced in the hot melt grinder and must therefore be discarded.
Another alternative to permitting the accumulation of hot parison is to purge the extruder with a material such as polyethylene which must not be kept in motion in the extruder so that the extruder can be shut down during adjustments or repair. The drawback of this method is that a substantial amount of PVC and polyethylene become intermixed, unusuable and must be discarded. The discarded plastic materials are non-reusable waste. Since each purging may require as much as 40 to 50 lbs. of plastic material this alternative is relatively expensive.
The magnitude of the above discussed waste problem is best illustrated by the fact that under a normal, three-shift operation of a conventional blow molding machine there is an average yearly waste of about 14,000 lbs. of PVC and 14,000 lbs. of polyethylene per machine. At current prices of 47 cents and 30 cents per lb., respectively, for these materials this amounts to a yearly waste of about $10-11,000.